Presynaptic EGTA reduces pairing-specific facilitation. Paired training is indicated by an arrow(tetanus) and a large open triangle (5-HT). In both control (open circles) and paired/control (open squares) groups, sensory neurons received injection of vehicle solution. The 5-HT-alone (small open triangles) data were obtained from the second sensory neurons in the cultures that did not receive any injection but only 5-HT puffs. For this and subsequent figures, *p < 0.05, **p < 0.01, and ***p < 0.001 versus control; #p < 0.05, ##p < 0.01, and ###p < 0.001 versus paired/control by Fisher’s PLSD tests. The average amplitudes of EPSPs on trial one, 30 min after impalement, were 12.7 ± 2.1 mV (control), 22.5 ± 6.5 mV (paired/control), 12.2 ± 2.5 mV (paired/EGTA), and 19.1 ± 3.5 mV (5-HT), not significantly different from each other [F_(3,35) = 1.39; p = 0.2625].